The present invention relates in general to voltage regulators and, more particularly, to low voltage regulators using the threshold voltage of Metal Oxide Semiconductor Field Effect Transistors (MOSFET) to regulate the output potential.
Most, if not all, electronic devices require at least one Direct Current (DC) voltage for proper operation. Many portable electronic devices require two or more levels of DC potential to operate properly, such as Compact Disc (CD) or Moving Picture Experts Group Audio Layer-3 (MP3) format players, for example. Each DC potential is derived from a battery power supply or an Alternating Current (AC) to DC conversion power supply. Multiple voltage regulators, therefore, are employed to generate the various DC potentials required by the CD or MP3 players. The voltage regulators are referenced to the single battery or AC-DC power supply input, where a separate voltage regulator is required to generate each level of DC potential required by the portable electronic device.
FIG. 1 illustrates prior art regulator 10 utilizing bipolar transistor 14 as the pass element and zener diode 16 as the reference element. In operation, an input voltage is applied to terminal Vin, which is larger than the breakdown voltage, Vz, of zener diode 16. The base terminal potential of transistor 14 is held relatively constant at Vz, causing transistor 14 to be conductive. The output voltage, Vout, of prior art regulator 10 is equal to Vzxe2x88x92Vf, where Vf is the emitter-base potential of transistor 14. Resistor 12 limits the current flow conducted by zener diode 16 when operating at the breakdown voltage Vz.
Zener diode 16 operates in the reverse biased, breakdown region and as a result, a reverse leakage current is present due to minority carriers being present within the vicinity of the depletion region. Prior art regulator 10 is subsequently used in applications requiring greater than 5 volt regulated output voltages, since the reverse leakage current is minimal in relatively high output voltage ranges. Applications requiring less than 5 volts, however, increases the amount of reverse leakage current conducted by zener diode 16, which precludes their use in low voltage applications ( less than 5 volts), where leakage current conducted by zener diode 16 is generally in the milliamp (mA) range.
Hence, there is a need for a simple, low voltage reference, exhibiting low current drain, thereby decreasing power dissipation and increasing efficiency.